Wind power is desirable because it is renewable and typically cleaner than fossil fuel power sources. Wind turbines capture and convert the energy of moving air to electric power. However, they do so unpredictably and often during low power demand periods when the value of electric power is substantially lower than during peak demand periods. Without a way to achieve certainty of delivery during peak demand periods (also known as “firm” power), and without a way to store low-value off-peak power for release during high-value peak periods, the growth of wind power (and other intermittent renewable power sources) may be constrained, keeping it from reaching its full potential as part of the world's overall power generation portfolio.
Even in the most wind-rich locations, the duration, daytime vs. nighttime availability, and the wind's “density” are unpredictable. Without power storage, there is no certainty of the wind providing firm power during any selected peak (or off-peak) consumption period. Thus, there is a need for a utility-scale power storage system that can guarantee firm power output during peak consumption periods, particularly where there is an increasing reliance on (or adoption of) intermittent renewable power sources.
Operating wind turbines (or other intermittent renewable power assets) adjacent to and in conjunction with a natural gas—(NG) fired turbine can yield 100% certainty of power, because the NG turbine can “back up” the wind. However, that approach will yield a reduced environmental rating, based on the hours of operation for the NG turbine and may be economically unfeasible because the two power output systems need to be fully redundant, and thus capacity utilization and economic return-on-assets is diminished. Most importantly, neither a standard wind farm nor a back-up NG turbine(s) can “store” the wind power that may be widely available during the off-peak periods.
A further disadvantage of intermittent power sources such as wind is that they can cause system “balance” problems if allowed onto the transmission grid, which is a major hurdle for new (particularly renewable) power generation sources to clear. Thus, there is a need for wind power storage and release systems having improved efficiency and predictability, while remaining cost-effective to deploy on a large scale (i.e., “utility scale”).
A disadvantage of other types of utility-scale power sources is that they produce large and unnecessary amounts of power during off-peak periods or intermittently. For example, base-load coal-fired and nuclear power plants continue to produce power at night at approximately the same rate of generation that they do during the day, even though far less power is needed at night than during the day. Cost-effective utility-scale power storage solutions that can release power during peak demand periods would dramatically increase the value of existing base-load power generation assets because power producers can typically charge significantly more for power sold during the day versus selling it during the night. Moreover, such a storage and release system would diminish the need to add new base-load coal or nuclear power capacity to meet growing power demand. It would also lower the consumption of fossil fuels and nuclear fuel by producing more “usable” kilowatts of power per unit of fuel consumed to produce the power, contributing significantly to the reduction of air pollutants, carbon emissions and hazardous/radioactive waste that result from today's base-load power plants. Such a utility-scale storage and release system would also contribute greatly to the more rapid adoption and broader deployment of other renewable power sources that produce power intermittently or during off-peak periods, such as solar, landfill gas anaerobic digesters, wave/tidal, and waste-to-energy power generation systems, among others.
Another major disadvantage of existing power systems is that transmission lines often become “clogged” or overloaded (particularly as it relates to transmitting intermittent power, like wind power), and transmission systems can become unbalanced. One existing solution for overloaded transmission lines is transferring power by “wheeling,” which is the delivery of a specific quantity of power to each end-user, allowing any “power product” to enter the power transmission system and be used to “balance” any other product that was removed from the system. A disadvantage of using current storage systems for wheeling is that power production occurs during all hours (most of which are not peak demand hours), and does not substantially overlap with peak demand hours. Another disadvantage is that transmission of power, which occurs at all hours (most of which are not peak demand hours), also does not substantially overlap with peak demand hours.
Thus, there is a need for an energy storage and release system that can help to alleviate the problem of transmission lines becoming overloaded by allowing power to be stored near the point of production or near the point of consumption (or at any point in between), which also allows the power that will be stored to be moved across transmission systems during off-peak transmission periods (such as at night) and thereby reducing the power “traffic” that moves across transmission lines during peak demand periods.
The few utility-scale power storage systems that exist today (or have been proposed previously) also have major disadvantages such as inefficient heat and cold recovery mechanisms, particularly those that require multiple systems for hot and cold storage media. Another disadvantage is extra complexity in the form of many expanders and compressors often on the same shaft with “clutches” that allow some front-end elements to be disconnected from the back-end elements on the same shaft. Thus, there is a need for more efficient hot and cold storage recovery mechanisms and simpler, more efficient compression and expansion systems that allow the compressors to operate independently of the hot gas expanders and do not require complex clutch systems to turn on and off.
Some existing power plants use a simple cycle gas turbine with a recuperator, where a front-end compressor is on the same shaft as the hot gas expander that compresses the inlet air. However, in that configuration some 63% of the power output is devoted to compressing inlet air. Thus, there exists a need for a system that can reduce the amount of power output required to compress inlet air.
Therefore, there exists a need for a system that can provide certainty and a firm, consistent energy output from any power source, particularly intermittent power sources such as wind. There is also a need to provide a convenient storage system for power that can be used in connection with power generation sources, particularly intermittent power sources such as wind turbines, but also for any power source that generates large amounts of power during off-peak periods. There is a further need for a power storage and release assembly having more efficient hot and cold recovery mechanisms and simpler, more efficient, compression and expansion systems.